1. Field of the Invention
The present invention relates to a method, system and program for a data storage environment.
2. Description of the Related Art
In prior art virtual tape storage systems, hard disk drive storage emulates tape drives and tape cartridges. For instance, host systems perform input/output (I/O) operations with respect to tape by performing I/O operations with respect to a set of hard disk drives that emulate the tape storage. In the prior art International Business Machines (IBM) Magstar** Virtual Tape Server, one or more virtual tape servers (VTS) are each integrated with a tape library comprising numerous tape cartridges and tape drives and have a direct access storage device (DASD) comprised of numerous interconnected hard disk drives. The DASD functions as a cache to volumes in the tape library. In VTS operations, the virtual tape server intercepts the host's requests to access a volume in the tape library and returns data for such requests from the DASD. If the volume is not in the DASD, then the virtual tape server recalls the volume from the tape drive to the DASD. The virtual tape server can respond to host requests for volumes in tape cartridges from DASD substantially faster than responding to requests for data from a tape drive. In this way, the DASD functions as a tape volume cache for volumes in the tape cartridge library. **AIX, ESCON, Magstar, and RS/6000, OS/390, Tivoli are registered trademarks of IBM.
Two virtual tape servers can be combined to create a peer-to-peer virtual tape server system. In a peer-to-peer virtual tape server, two virtual tape servers, each integrated with a separate tape library and DASD, can provide access and storage for the same data volumes (i.e., peer-to-peer environment). In such peer-to-peer VTS systems, if an operation to recall a file from one virtual tape server subsystem and tape library fails, then the file may still be recalled from the other virtual tape server subsystem and tape library. This redundant architecture provides greater data and tape availability and improved data shadowing in the event a tape or VTS in one subsystem is damaged. Upon receiving a write request, a virtual tape controller connected to both virtual tape servers, will write the logical volume to only one of the virtual tape servers. In the VTS peer-to-peer system, virtual tape controllers receive host I/O requests and direct such requests to one of the VTS subsystems (an example of a virtual tape controller is the IBM AX0 Virtual Tape Controller (“VTC”) which acts as an intelligent switch between the two virtual tape servers). Then, the VTC copies the logical volume from the virtual tape server that last received the write to the other virtual tape server. Many peer-to-peer configurations include more than one virtual tape controller to provide redundancy and no single point of failure. When one of the virtual tape controllers is off-line all host activity is directed to the remaining one or more virtual tape controllers. All virtual tape controllers in a Peer-to-Peer VTS, as a group elect one of the VTSs to be the focal point or master VTS of the Peer-to-Peer VTS and control the switchover to the other VTS, if the master VTS fails. The virtual tape controllers combine the status of the underlying physical libraries to provide a logical view of the peer-to-peer virtual tape system to the host. The virtual tape controllers are also referred in the art as intermediate controllers.
When one of the virtual tape servers in a peer-to-peer virtual tape server configuration is taken off-line for maintenance, routine service, or even a system upgrade, all activity will be directed to one of the remaining virtual tape servers. When the virtual tape server that was taken off-line returns to service, each of the virtual tape controllers will resume copy operations to bring the logical volume copies in both virtual tape servers back into synchronization.
A token is a data stored in the databases of both distributed VTSs, and is used to keep track of the level and status of each data set (e.g. a logical volume, also referred to as a virtual volume) within the Peer-to-Peer VTS. The tokens guarantee data integrity in the Peer-to-Peer VTS. In addition, token data is stored with each virtual volume when written onto physical tape. The token data for a virtual volume includes information that indicates whether a virtual volume resides in the DASD (tape volume cache) of the VTS or has already been copied to a stacked physical cartridge. The virtual tape controllers direct the mount request to the VTS with the cached copy of the volume in DASD to improve recall efficiency.
The token data for a virtual volume further includes information that indicates whether the distributed VTS contains a valid copy of a virtual volume. This prevents using an outdated version of a virtual volume. The process to compare tokens stored in each VTS to determine required copy updates and token synchronization is called token reconciliation. Token reconciliation is performed at power-up and at periodic intervals by the virtual tape controllers and may involve examining the internal fields of one or more data structures reflecting a token.
Notwithstanding, there is a need in the art for improved techniques for managing the tokens and performing token reconciliation.